Enabling manipulation and observation of cells on a single cell level has been a major focus of microfluidics research for the past two decades. Such technologies would allow quantification of the degree of heterogeneity within a population of cells, which would otherwise be characterized by an averaged result obtained from analyzing the lysate of many cells. This is useful in screening populations of cells for a rare mutation, useful for diagnosis and genotyping of human disease and in commercial applications where mutations are induced to improve a product, such as in the agricultural industry.
Microfluidic droplets are useful to contain single cells in order to isolate the assays and characterization done on each cell. The products from an assay or reaction are kept contained in a small volume, which facilitates their detection by fluorescence or other modalities. In addition, monodisperse droplets of nanoliter to picoliter volume can be produced rapidly using microfluidics, and analyzed or sorted on the same microfluidic platform. For these reasons, droplets have been applied to the screening of conditions for protein crystallization, screening of individual biological cells, screening of enzymes for directed evolution, and to construct a detailed, precise dose-response curve for pharmacology.